1. Field of the Invention
The present invention relates to an electrostatic holding apparatus, and more particularly to an electrostatic holding apparatus useful for a process performed during fabrication of semiconductor devices, liquid-crystal devices, etc., which apparatus can strongly attract and hold a conductive, semiconductive, or insulating object by means of electrostatic force and which enables easy attachment/detachment of the object.
2. Description of the Related Art
Recently, processes for fabricating semiconductor devices, liquid-crystal devices, etc.; especially processes of drying etching, ion implantation and vapor deposition, have been more and more automated and performed as dry processes. Under such circumstances, the number of fabrication processes performed under vacuum has increased.
Meanwhile, positional accuracy at the time of patterning has become more important since the diameters of silicon wafers and glass plates serving as substrates have increased and the degree of integration of circuits and the degree of fineness of patterns have increased. Therefore, vacuum chucks have been used to transport wafers or to attract and fix wafers. However, vacuum chucks have the following drawbacks. Vacuum chucks cannot be used under a vacuum environment because of impossibility of creating a pressure difference. Although vacuum chucks can be used under a non-vacuum environment, a sucked wafer undergoes a local distortion because the wafer is sucked locally, with the result that accurate positioning becomes difficult. Therefore, vacuum chucks are not suitable for recently developed processes for fabricating semiconductor devices and liquid-crystal devices.
Recently, as a device that has overcome the above-described drawbacks, there has been widely noticed and put into practical use an electrostatic holding apparatus which transports and/or attracts and fixes a wafer by means of electrostatic force. In a recent process for fabricating semiconductor devices and liquid-crystal devices, with an increase in the degree of fineness of devices, the flatness of wafers and glass plates serving as substrates has become more and more important. Therefore, employment of electrostatic holding apparatuses has been considered in order to perform a straightening correction for improving the flatness of wafers and glass plates.
Such a straightening correction for improving the flatness of wafers and glass plates requires a very strong electrostatic force. In order to meet such a requirement, there has been proposed a technique in which titania (TiO3) is mixed into an insulating material such as alumina in order to decrease the volume resistivity to thereby increase the electrostatic attraction force (see Japanese Patent Application Laid-Open (kokai) Nos. 62-94953, 2-206147, 3-147843, and 3-204924).
As described above, when alumina containing titania is used for an insulating dielectric layer of an electrostatic attraction portion, the volume resistivity of the insulating dielectric layer decreases, and a small current flows therethrough, so that an increased electrostatic force is generated due to the Johnson-Rahbek effect. However, since titania is a semiconducting substance, movement velocity of charges decreases, so that even when the volume resistivity is optimized, the response characteristics (time required for reaching saturated attraction force, time required for annihilation of residual attraction force) at the time of stopping application of voltage deteriorate. Consequently, an article becomes difficult to remove from the attraction surface of the electrostatic holding apparatus. This deterioration in the response characteristics becomes remarkable when the electrostatic holding apparatus is used at low temperature. Further, due to a difference in coefficient of thermal expansion between titania and alumina, cracks and/or pores are generated in sintered alumina, so that the withstanding voltage is low.
The present invention has been accomplished to solve the above-mentioned problems, and an object of the present invention is to provide an electrostatic holding apparatus in which the volume resistivity of an insulating dielectric layer of an electrostatic attraction portion is decreased to increase an electrostatic force, which does not suffer deterioration in the capability of allowing removal of an object at the time of stopping application of voltage, in which neither fine cracks nor pores remain in the insulating dielectric layer after sintering, and which is therefore excellent in terms of withstand voltage.
To achieve the above object, the present invention provides an electrostatic holding apparatus in which a voltage is applied to an conductive electrode covered with an insulating dielectric layer in order to cause the insulating dielectric layer to electrostatically attract an object, wherein the main component of the insulating dielectric layer is ceramic containing 0.1-30 wt. % of an atomic metal, and the volume resistivity of the metal-containing ceramic at 20xc2x0 C. is 108-1013 xcexa9xc2x7cm.
If the main component of the insulating dielectric layer is ceramic containing 0.1-30 wt. % of an atomic metal, the volume resistivity of the insulating dielectric layer at 20xc2x0 C. can be controlled easily and precisely to fall within the range of 108-1013 xcexa9xc2x7cm. As a result, it becomes possible to form an electrostatic attraction portion which produces a strong electrostatic force, while enabling easy removal of an object; which has a reduced coefficient of thermal expansion; in which neither fine cracks nor pores remain in the insulating dielectric layer after sintering; and which therefore has a high withstand voltage.
Preferably, the atomic metal is molybdenum or tungsten.
If ceramic containing 0.1-30 wt. % of Mo or W as an atomic metal is used, the volume resistivity of the insulating dielectric layer at 20xc2x0 C. can be brought into the range of 108-1013 xcexa9xc2x7cm. As a result, it becomes possible to form an electrostatic attraction portion which produces a strong electrostatic force, while enabling easy removal of an object; which has a reduced deference in coefficient of thermal expansion; in which neither fine cracks nor pores remain in the insulating dielectric layer after sintering; and which therefore has a high withstand voltage.
Preferably, the ceramic of the insulting dielectric layer is a ceramic material selected from the group consisting of aluminum nitride, aluminum oxide, silicon nitride, silicon oxide, zirconium oxide, titanium oxide, sialon, boron nitride, and silicon carbide, or a mixture of two or more kinds of ceramic materials selected from the group.
When the ceramic of the insulting dielectric layer is selected from the above-described group, the addition of the above-described single metal to the ceramic makes it possible to obtain an increased electrostatic force and to enhance heat resistance and plasma resistance. Further, since the difference in coefficient of thermal expansion between the ceramic and the atomic metal mixed therein can be decreased to a possible extent, neither distortions nor cracks are generated in the attraction surface of the electrostatic attraction portion after a sintering process, so that the withstand voltage characteristic is improved. As a result, defects such as distortions and cracks are reliably prevented from being generated in a substrate (e.g., semiconductor wafer or glass plate) that is attracted and held by means of the electrostatic holding apparatus.
In the electrostatic holding apparatus according to the present invention, the insulating dielectric layer of the electrostatic attraction portion for attracting an object is formed of ceramic containing 0.1-30 wt. % of Mo or W, and the volume resistivity of the insulating dielectric layer at 20xc2x0 C. is controlled to fall within the range of 108-1013 xcexa9xc2x7cm. Accordingly, the electrostatic holding apparatus produces a strong electrostatic force, while enabling easy removal of the object. Further, since the coefficient of thermal expansion of the insulating dielectric layer is substantially equal to that of AlN or SiC, neither fine cracks nor pores remain in the insulating dielectric layer after sintering, so that the electrostatic holding apparatus provides high performance and has excellent characteristics, including high withstand voltage. Therefore, the electrostatic holding apparatus of the present invention has a great industrial utility.